1. Field of the Invention
The present invention relates to an ink-jet recording head, and an ink-jet recording apparatus employing the ink-jet recording head.
2. Related Background Art
The ink-jet recording system as disclosed, for example, in Japanese Patent Application Laid-Open No. 54-51837, has characteristics different from other ink-jet recording systems in that the driving force of the discharging of liquid droplets is derived by application of thermal energy to the liquid. More specifically, in the ink-jet recording method disclosed in the above Laid-Open gazette, a liquid is heated by application of thermal energy to form a bubble, and the liquid droplet is discharged, by action of the force generated by bubble formation, through a discharge opening at the tip portion of the recording head to be allowed to deposit onto a receding medium to record information thereon.
The ink-jet recording head (hereinafter simply referred to as a "recording head") employed in the ink-jet recording system is provided with a liquid discharge portion. The liquid discharge portion generally comprises a discharge opening for discharging the liquid, a liquid path communicating with the discharge opening, and a heat-generating means provided in the liquid path for applying the thermal energy to the liquid. An example of the heat-generating means is an electrothermal transducer which comprises a lower layer for heat accumulation, a resistance layer having a heat-generating portion, a pair of wiring electrodes for supplying electricity to the resistance layer, and a protective layer for protecting the wiring electrodes against the ink.
From the standpoint of design of the recording head, the protective layer is preferably formed as thin as possible, or more preferably not formed, in order to transfer the thermal energy effectively to the ink. However, in conventional recording heads, the protective layer had to be formed thick on and around the border portion between the heat-generating portion and the wiring electrodes to protect the wiring electrode, because the wiring electrodes are formed thick to decrease the electric resistance with a large height of the electrode pattern.
On the other hand, the resistance layer is relatively thin in comparison with the wiring electrodes since the resistance layer has high electric resistance. Accordingly, the protective layer can be made thin at the heat-generating portion of the resistance layer (the region of the resistance layer which is between the pair of wiring electrodes and has no wiring electrode built up thereon).
Japanese Patent Application Laid-Open No. 60-236758 proposes formation of the protecting layer to be thin at the heat-generating portion. However, it does not specifically consider where the protective layer is to be thinned.
Japanese Patent Application Laid-Open No. 63-191645 discloses wiring electrodes provided beneath the resistance layer at an organic protective layer portion covering the heat-generating portion to decrease the temperature rise of the organic protective layer portion since the organic protective layer is less heat-resistant. However, this arrangement is employed in consideration of the durability of the protective layer, but the relation with the resistance layer is not considered.
Japanese Patent Application Laid-Open No. 55-126462 discloses a layer constitution having no protective layer. The resistance layer in such a layer constitution should have sufficient ink resistance, having excellent electrochemical properties at a high temperature, and being resistant against cavitation caused on bubble disappearance. The suitable material for the resistance layer having the above properties include Al-Ta-Ir disclosed in Japanese Patent Application Laid-Open No. 1-46769, and Ta-Ir disclosed in Japanese Patent Application Laid-Open No. 2-55131.
However, in the recording head which has a protective layer thinner at the heat-generating portion, the discharge durability varies depending on the thickness of the protective layer, and may be inferior in discharging characteristics.
The inferior discharging characteristics are found to result from the causes below by failure analysis. The first cause is that a crack appears at the thin portion of the protective layer, and ink penetrates through the formed crack to react with the resistance layer at a high temperature to destroy it. The second cause is that the thermal stress of the protective layer against the resistance layer breaks the resistance layer at the thin portion of the protective layer. More specifically, the protective layer is formed relatively thicker on the wiring electrode layer to cover the level difference of the electrode pattern, and is formed as thin as possible on the heat-generating portion. Therefore, the thick region and the thin region of the protective layer exist on the heat-generating portion on and around the pattern boundary of the heat-generating portion and the wiring electrode (see FIGS. 9A and 9B). When the heat-generating portion of the resistance layer generates heat, heat expansion difference between the thick region and the thin region of the protective layer impose stress between those regions to cause cracking of the protective layer, or to damage the lower resistance layer to destroy finally the resistance layer by high-temperature reaction with ink having penetrated through the crack of the protective layer. Otherwise the resistance layer under the boundary of the thick portion and the thin portion of the protective layer may be broken by the aforementioned stress of protective layer.
In particular in the present invention, an ink-jet system is employed which discharges ink by pressure of film-boiling of the ink, and the heat is generated abruptly in a very short time in the heat-generating portion of the resistance layer to impose great heat stress to the upper protective layer. The stress is stronger at the portion where the thickness of the protection is changed.
On the other hand, in the similar ink discharge test using a recording head in which the heat-generating portion of the resistance layer is brought into direct contact with the ink (namely, no protective layer on the heat-generating portion, see FIGS. 10A and 10B), the durability varies around the boundary between the protected and the unprotected regions, similarly in the recording head having a protective layer.
As the result of the failure analysis, the first cause is the large difference of stress in the protective layer between the protected and the unprotected regions of the resistance layer on heat generation to break the resistance layer, similarly as the aforementioned second cause. The second cause in this case is electrochemical reaction. In particular, when the resistance layer is made thinner to raise the sheet resistance for weaker current drive for the purpose of using an inexpensive driving element, the potential difference in the resistance layer becomes larger, which accelerates the electrochemical reaction to cause breakdown of the resistance layer in a short time.
The breakdown of the resistance layer by electrochemical reaction is considered below for a layer constitution in which the heat generating portion is brought into direct contact with the ink. The breakdown of the resistance layer by the electrochemical reaction is considered to result from the causes below:
(1) Attack by alkali metal ions against the negative electrode portion: The resistance layer and the heat accumulation layer are liable to be attacked by electrochemical reaction especially at the end portion of the resistance layer pattern, and PA1 (2) Dissolution of the resistance layer at the positive electrode portion. PA1 (i) Voltage: A higher driving voltage for the resistance layer increases the potential difference in the heat-generating portion, accelerating the electrochemical reaction. PA1 (ii) Temperature: A higher temperature naturally accelerates the reaction, since the electrochemical reaction is a kind of chemical reaction. This depends on the ratio of the driving voltage to the bubble formation voltage and the driving pulse width. PA1 (iii) Heating time: The progress of the electrochemical reaction depends on the heating time within one pulse, or the driving pulse width. PA1 (iv) Kind of ink: The electrochemical reaction is naturally affected by the ion species contained in the ink. PA1 (v) Material and thickness of resistance layer: The electrochemical reaction naturally depends on the material of the resistance layer. The time passed before the breakdown depends on the layer thickness. The larger the thickness, the longer the time passed before breakdown.
The electrochemical reaction is accelerated by the factors below:
The progress of the electrochemical reaction varies with the above causes. In particular, in weaker electric current drive with a less expensive driving element to reduce cost, a higher sheet resistance is required for the resistance layer, which lowers the discharge durability.
The lower durability at the higher sheet resistance is considered as follows. The higher sheet resistance increases the potential difference in the resistance layer to accelerate the electrochemical reaction. The less thickness of the resistance layer results in poorer anti-electrochemical reaction properties. These two causes can lower the ejection durability.
Furthermore, the electrochemical reaction is accelerated by various factors such as a higher driving voltage with a certain pattern design of the resistance layer; higher maximum temperature of the resistance layer owing to variation in production of the recording heads at a driving voltage uniformized for cost reduction; and use of various ink for various recording paper. Therefore, a layer material and a layer constitution is required which are more stable electrochemically.
As described above, a measure is required to meet the change of the protective layer thickness on the heat-generating portion in order to improve the discharge durability irrespectively of the presence or absence of the protective layer on the heat-generating portion of the resistance layer.